The invention is suitable for suction engines and for supercharged engines. Thus, as regards supercharged internal-combustion engines, it is already known, for example from EP-A-0,072,059, to arrange an exhaust-gas particle filter in the high-pressure part of the exhaust-gas system in front of a gas-dynamic pressure-wave machine. If clogging of the exhaust-gas particle filter occurs under partial load, its pressure loss presents an obstacle to gas exchanges within the engine system, thus resulting in a reduction in effective power. The vehicle driver usually compensates the performance loss by supplying a greater amount of fuel. When the vehicle driver demands sufficient power from the vehicle, the exhaust-gas temperature rises sharply and the exhaust-gas particles deposited in the filter are burnt off automatically.
Where passenger vehicles are concerned most trips are usually made under low-load conditions, and in this case it is expedient to ensure that there is sufficient burn-off in the fairly rare high-load periods. Drivers who demand very little power from the vehicle must be borne in mind here.
In addition, the reaction, that is to say the regeneration of the filter, does not take place immediately. The burn-off rate depends very greatly on the temperature of the particles, on the oxygen concentration in the exhaust gas and on the quantity of particles present. The burn-off rate can be increased by means of catalytic coating of the exhaust gas particle filter and by means of additives in the fuel.
Operating phases with a high engine load usually follow those with a low load. Such conditions arise when the vehicle accelerates after stopping at a traffic light or reaccelerates after a deceleration phase in the traffic flow. In the low-load phase, the exhaust gases cool the particle filter and the particles to below 200.degree. C. Although the after-cylinder exhaust-gas temperature increases very rapidly to above 500.degree. C. during acceleration, nevertheless the filter material and soot particles must first be heated to a temperature at which burn-off can take place quickly enough. This typically lasts approximately ten seconds. After this time, the accelerator is frequently already released again. This shows how difficult it is to ensure that the exhaust-gas particle filters are not unduly clogged in any driving conditions.
There is also the danger, here, that the exhaust-gas particle filter will become very heavily laden in which case the filter may de damaged as a result of an over-intensive burn-off in a rare but relatively long high-load phase. It is therefore necessary to find systems in which the exhaust-gas particle filters burn off as frequently as possible and never become too heavily laden. This also reduces the attendant pressure drop in the exhaust-gas particle filter.
In engines with supercharging units operated with exhaust gas, in which the exhaust-gas particle filter is connected between the engine outlet and gas inlet of the supercharging unit, there is a further problem in that the acceleration behaviour is inadequate. This is explained as follows: although, as mentioned above, the after-cylinder exhaust-gas temperature jumps immediately to above 500.degree. C. during acceleration, nevertheless the exhaust gases are first cooled to values of around 200.degree. C. in front of the supercharging unit because of the large cold mass of the exhaust-gas particle filter. When the temperature is so low, the enthalpy gradient which can be utilized is only slight and the supercharging effect likewise remains slight. In this case, the engine does not deliver much more power than if it were not supercharged, and because of this supercharging is pointless.
This leads at once to the idea of increasing the back-pressure in the exhaust by reducing the throughflow crosssection of the supercharging unit. However, in exhaust-gas turbo-suprchargers with a fixed turbine cross-section, this is impossible in practice, if acceptable levels of efficiency are to be achieved during normal operation. In turbo-superchargers with a variable turbine geometry and in gas-dynamic pressure-wave machines, the absorption capacity can be reduced. However, the counterpressure in the exhaust rises to around one bar above the supercharging pressure. The effective mean pressure of the engine process is lowered by this amount, and the improvement in driving performance is limited, quite apart from the associated disadvantage with regards to consumption.
The exhaust-gas particle filter between the engine outlet and the gas inlet of the supercharger thus presents a serious problem in terms of acceleration. Arranging the exhaust-gas particle filter after the supercharger has hitherto been unsuccessful, because the temperatures are lower there and there is no reliable burn-off.
The acceleration power can be improved somewhat by over-fuelling. This means that the injection rate is set higher than is normally permissible because of the generation of soot. The soot which arises is mostly trapped in the exhaust-gas particle filter. However, this measure is limited, since the exhasut-gas particle filter otherwise becomes contaminated too quickly. There is therefore the additional requirement that the supercharger should deliver supercharging pressure rapidly and allow a further increase in the mean pressure of the engine, without an extremely low air excess. To achieve this, the exhaust-gas temperature in front of the supercharger must rise to 300.degree. to 400.degree. C. in approximately one second. A supercharging pressure ratio of 1.5 can be obtained in this way.
A process and a device of the type mentioned in the introduction were presented at a symposium on motor-vehicle diesel engines held on Oct. 3rd to 4th, 1983 at the Wuppertal Technical Academy in Germany. The measure adopted to reduce the requirement for secondary energy, necessary to start regeneration and carry out regeneration, is the bypass circuit. This has to be regulated as a function of the load and speed in the characteristic diagram. In this respect it has been stated that, in a specific power range of a supercharged engine, some of the exhaust gases, for example half, could be guided directly in front of the supercharger by the bypass at least during the acceleration phase. The desired mixing temperature of 300.degree. to 400.degree. C. is then obtained immediately there. The bypass must open one to two bars below the mean pressure which can be reached in non-supercharged running. Under normal operating conditions, for example in the U.S.-City cycle, a passenger car drives above this limit only a small percentage of the time. However, more soot is generated under a high load. Nevertheless, only a fraction of the soot is generated above the said limit. Even if some of it is not filtered, nevertheless, over all, the statutory value can be achieved. The extremely strict regulations in California could be an exception to this.
If the vehicle operates partly under bypass conditions in the acceleration phase, the disadvantage of this is that in the very period in which, according to experience, diesel engines produce the most smoke, some of the exhaust gases are not filtered. The possibility of slight over-fuelling during acceleration is forestalled.